It is well known that the resistivity of many conductive materials change with temperature. The resistivity of a positive temperature coefficient (PTC) conductive material sharply increases as the temperature of the material increases over a particular range. Many crystalline polymers, made electrically conductive by dispersing conductive fillers therein, exhibit this PTC effect. These polymers generally include polyolefins such as polyethylene, polypropylene and ethylene/propylene copolymers. At temperatures below a certain value, i.e., the critical or trip temperature, the polymer exhibits a relatively low, constant resistivity. However, as the temperature of the polymer increases beyond the critical point, the resistivity of the polymer sharply increases. Compositions exhibiting PTC behavior have been used in electrical devices as over-current protection in electrical circuits comprising a power source and additional electrical components in series. Under normal operating conditions in the electrical circuit, the resistance of the load and the PTC device is such that relatively little current flows through the PTC device. Thus, the temperature of the device (due to I.sup.2 R heating) remains below the critical or trip temperature. If the load is short circuited or the circuit experiences a power surge, the current flowing through the PTC device increases greatly. At this point, a great deal of power is dissipated in the PTC device. This power dissipation only occurs for a short period of time (fraction of a second), however, because the power dissipation will raise the temperature of the PTC device (due to I.sup.2 R heating) to a value where the resistance of the PTC device has become so high, that the current is limited to a negligible value.
The new current value is enough to maintain the PTC device at a new, high temperature/high resistance equilibrium point. The device is said to be in its "tripped" state. The negligible or trickle through current that flows through the circuit will not damage the electrical components which are connected in series with the PTC device. Thus, the PTC device acts as a form of a fuse, reducing the current flow through the short circuit load to a safe, low value when the PTC device is heated to its critical temperature range. Upon interrupting the current in the circuit, or removing the condition responsible for the short circuit (or power surge), the PTC device will cool down below its critical temperature to its normal operating, low resistance state. The effect is a resettable, electrical circuit protection device.
Conductive polymer PTC compositions and their use as protection devices are well known in the industry. For example, U.S. Pat. Nos. 4,237,441 (Van Konynenburg et al.), 4,304,987 (Van Konynenburg), 4,545,926 (Fouts, Jr. et al.), 4,849,133 (Yoshida et al.), 4,910,389 (Sherman et al.), and 5,106,538 (Barma et al.) disclose PTC compositions which comprise a thermoplastic crystalline polymer with carbon black dispersed therein. Conventional polymer PTC electrical devices include a PTC element interposed between a pair of electrodes. The electrodes can be connected to a source of power, thus, causing electrical current to flow through the PTC element.
However, in prior conductive polymer PTC compositions and electrical devices employing such compositions, the polymer PTC composition has been susceptible to the effects of oxidation and changes in resistivity at high temperatures or high voltage applications. This thermal and electrical instability is undesirable, particularly when the circuit protection device is exposed to changes in the ambient temperature, undergoes a large number of thermal cycles, i.e., changes from the low resistant state to the high resistant state, or remains in the high resistant (or "tripped") state for long periods of time.
Further, in electrical devices employing prior conductive polymer PTC compositions, poor physical adhesion (i.e., poor ohmic contact) between the PTC composition and the electrodes has resulted in an increased contact resistance. As a result, PTC devices employing these prior compositions have had high initial or room temperature resistances, thus, limiting their applications. Attempts to overcome this poor ohmic contact in prior PTC devices have generally focused on changes to the electrode design. For example, U.S. Pat. No. 3,351,882 (Kohler et al.) discloses a resistive element composed of a polymer having conductive particles dispersed therein and electrodes of meshed construction (e.g., wire screening, wire mesh, spaced apart wire strands, or perforated sheet metal) embedded in the polymer. Japanese Patent Kokai No. 5-109502 discloses an electrical circuit protection device comprising a PTC element and electrodes of a porous metal material having a three-dimensional network structure.
Other attempts at improving ohmic contact in PTC devices have included chemically or mechanically treated electrodes to provide a roughened surface. For example, U.S. Pat. Nos. 4,689,475 and 4,800,253 (Kleiner et al.), and Japanese Patent No. 1,865,237 disclose metal electrodes having chemically or mechanically treated surfaces to enhance surface roughness. These treatments include electro-deposition, etching, galvanic deposition, rolling or pressing. These treatments, however, increase the number of processing steps and increase the overall cost of the PTC device.